High-frequency heating applicators



Feb. 26, 1957 Filed March 26, 1954 H R. WARREN HIGH-FREQUENCY HEATINGAPPLICATORS 2 Sheets-Sheet l Feb. 26, 1957 R WARREN 2,783,349

HIGH-FREQUENCY HEATING APPLICATORS Filed March 26, 1954 2 Sheets-Sheet 2Fig, 7

United States Patent 2,783,349 HIGH-FREQUENCY HEATING APPLICATORS HenryR. Warren, Columbus, Ind., assignor to National Cylinder Gas Company,Chicago, 111., a corporation of Delaware Application March 26, 1954,Serial No. 419,07 0 21 Claims. (Cl. 219-1055) This invention relates tohigh-frequency heating and particularly to resonant applicatorsespecially suited for rapid dielectric heating of large area loads, suchas foamrubber mattresses, Wallboard panels, groups of sand cores orplastic preforms, and the like, and other similar applications ofhigh-frequency heating, wherein it frequently is required, or at leastdesirable, that the hot heating electrode be of large area, i. e., inthe order of several feet in one or both face dimensions.

This application, a continuation-in-part of my application Serial No.138,628, filed January 14, 1950, and now abandoned in favor of mycontinuation-in-part applica tion Serial No. 419,633, filed March 26,1954, has claims directed to subject matter divided from my parentapplication Serial No. 138,628. My aforesaid applications contain a moredetailed discussion, included herein by this reference, of thestructural and operating characteristics of resonant applicators of thekind to which the present invention is particularly directed.

In general, such applicators comprise relatively large electrodestructures electrically interconnected through conductive structurewhich at least in part has substantial inductance cooperative withcapacity-means including the capacitance between said electrodestructures to form a resonant circuit device, and a power transfercoupling loop disposed in position to be traversed by a high-frequencymagnetic field encircling a part of said interconnecting conductivestructure. More particularly, in the preferred form of such applicators,the interconnecting structure includes low-resistance, low-reactanceconductive walls of a shielding enclosure which completes the resonantcircuit of said device and serves to confine said magnetic field andalso the electric field produced between said electrode structures, atleast one of the electrode structures being electrically interconnectedwith wall structure of the enclosure through a leg or fin-like elementwhich projects into the enclosure and in which a substantial part of theinductance of the resonant applicator is concentrated, and the saidcoupling loop being arranged to be traversed by the magnetic fieldencircling the inwardly projecting leg or fin-like element. Such loopmay serve to provide for excitation of the applicator by, preferably butnot limited to, a self-excited oscillator having the loop in its anodecircuit.

In accordance with one important aspect of the present invention, theinwardly projecting leg or fin-like element is made in the form of ahollow structure, with numerous resulting advantages as will be pointedout hereinafter.

Such hollow inductance structure may be of rectangular,

round, octagonal or other suitable cross-sectional outline, and may beeither similar to or unlike the outline of the associated electrodestructure, depending upon the particular heating application encounteredand structural requirements of the applicator as a whole. Moreover, theleg or fin-like element may be dimensioned and shaped so as to obtain anapplicator inductance which has high Q. In addition, by properdimensioning and shaping of the fin-like inductance structure, and byproper location thereof either symmetrically or unsymmetrically withrespect to the associated electrode structure, voltage gradients invarious directions along large heating electrodes may be minimized orcontrolled without the necessity for resorting to stubbing with itsattendant disadvantages and complications. When the spacing between theelectrode structures is to be variable to accommodate therebetween loadsof different heights, the hollow inductance structure may be extensibleand to such end it may comprise, at least in part, metal straps or websor other suitable flexible conductive elements.

The hollow inductance structure, in accordance with another importantfeature of the present invention, may be made so as to provide ashielded compartment in which associated mechanical or electricalapparatus, such as electrode raising devices, oscillator circuitcomponents, and the like, may be disposed in substantial isolation fromthe intense high-frequency magnetic and electric fields of theapplicator.

The invention further resides in high-frequency heating applicatorshaving features of construction and arrangement hereinafter describedand claimed.

For a more detailed understanding of the invention and for illustrationof various embodiments thereof, reference is made to the accompanyingdrawings in which:

Fig. 1 is a sectional view of a press applicator;

Fig. 2- is a perspective view, partly broken away, of the applicatorshown in Fig. 1;

Fig. 3 is a perspective view, with parts broken away, of a modificationof the applicator shown in Figs. 1, 2;

Fig. 4 is a sectional view of a further modification;

' Fig. 5 is a perspective view, with parts broken away or omitted, ofthe applicator of Fig. 4;

Fig. 6 is a perspective view, with parts broken away or omitted, of amodification of the applicator of Figs. 4, 5;

Fig. 7 is a sectional view of an inverted press-applicator; and

Figs, 8 and 9 are sectional views of other resonant applicators ofnon-press type.

The dielectric-heating press-applicator 10A shown in Figs. 1 and 2 isparticularly suited for production of laminated plywood sheets ofsubstantial area (for example, 4 feet by 8 feet sheets), for productionof large panels by edge-bonding of wooden strips, and for like purposesrequiring large heating electrodes and dissipation of many kilowatts ofradio-frequency power in the dielectric load. The press frame is in theform of an elongated metal tunnel 11A of substantially rectangularcross-section.

A plurality of pressure-applying devices, specifically cylinders 18Asupplied from pressure lines 20A, are spaced lengthwise of the tunnel11A with their plungers or rams 17A attached, Without interposition ofinsulation, to correspondingly spaced regions lengthwise of theelongated, movable platen 16A which serves as the hot electrode of theapplicator. In this modification, as well as in others herein described,the bottom of the tunnel may itself serve as the lower or cold heatingelectrode 15A. Alternatively, the cold electrode may be an auxiliaryconductive member, movable or stationary, conductively connected orotherwise coupled to the tunnel wall structure.

For applying lateral pressure to the heating load, as in edge-bonding ofwooden strips 19A, the applicator frame may be provided with a secondseries of pressure-applying devices, exemplified by cylinders A18supplied from pressure-lines A20, having plungers or rams for applyinglateral pressure to the work strips 19A either directly or through aninterposed filler block 48A.

The tunnel walls may be relatively thin sheet metal reinforced, assuggested by frame members 47A, to provide the strength and rigiditynecessary to resist deformation by the pressure. The dimensions anddisposition of the reinforcing members may vary widely to suit thepressure-resisting requirements of difiere'nt installations.

In edge bo'n'din'g, the pressure applied horizontally is quitesubstantial: the pressure applied vertically is relatively light butsufiicient to prevent buckling by the applied side pressure.

The row of plungers .17A is within a hollow inductance structure or fin13A defined by the two rows of straps 49A extending lengthwise ofelectrode 16A on opposite sides of the plungers with each strapconnected from electrode 16A to the top wall of the tunnel. Morespecifically, the upper ends of the strapsmay be connected to the bars56A attached to the top walls of housing 11A and the lower ends of thestraps may be connected to bars 57A attached to the upper face ofelectrode 16A..

.In the arrangement of Figs. 1 and 2, all, or practically all, of theinductance of the reentrant resonant applicator is concentrated in thehollow vertical fin structure 13A afiorded by the parallel, elongatedrows of straps 49A, and the applicator is resonant at a frequencypredominantly determined by the inductance of the fin structure and thecapacitance between the heating electrodes 15A,

16A. These straps 49A are preferably of relatively ii areas that may beassociated therewith as in certain of the I embodiments disclosed in myaforesaid copending application. The space within the hollow fin (i. e.,between therows of straps 49A) is effectively isolated from thesehigh-frequency fields by the rows of straps 49A and the shielding of theinwardly extending end of the fin or inductance structure 13A by theelectrode 16A. The spaced rows of straps define a substantiallyfield-free or isolated compartment for the plungers 17A. Thus, theelectrode-actuating members or plungers (17A) need not includeor becomprised of insulating material, nor do they need be of metal havinghigh electrical conductivity and in good electrical contact with thehousing wall.

The high-frequency resistance of the hollow inductor 13A and of the hotelectrode 16A is very low. The resistance of the remainder of thecurrent path afforded by the extensive area of the wall structure of thetunnel is also very low so that, as in all applicators herein described, the Q is very high despite the high ratio of capacitancetoindu'ctance. i

, Becauseof their high-frequency, the circulating currents, which may beover a thousand amperes, are practically confined to the inner surfacesof the applicator housing and consequently all external surfacesincluding the cylinders 18A, A18 and the pressure-lines 20A are atground potential. The applicator housing serves as a radio-frequencyshield for theinternal components which are atvery high radio-frequencypotential. Therefore the radiation losses are low, which minimizes radiointerference and further contributes to high Q of the tunnel applicator.

Another and very important advantage of this .type of applicator is itsunique suitability for dielectric heating of load materials having verylowpower-factor including foam rubber, extruded rubber hose and gaskets,while also being suitable for heating high power-factor materials, suchas wood.

The applicator 10B of Fig. 3 is of construction similar to that of Figs.1, 2 except that the two rows of straps 49A are replaced by two widesheets 49B on opposite sides of the row of plungers 17B with each sheetconnected from electrode 153 to thetop Wall of housing 113 so to definean elongated hollow fin inductance 13B within whose fieldfree space theelectrode-actuating plungers 17B, or equivalent, are disposed. Thesheets 49B are preferably of relatively springy metal of high electricalconductivity, such as beryllium-copper or some other springy metalcoated with copper or other metal of high conductivity. Thismodification need not be further described as the description of Figs l,2 is similarly applicable thereto.

In the modified term of press applicator shown in Fig. 4, theconstruction is similar to that shown in Figs. 1 to 3 except that thevertical cylinders are within the applicator housing and serve as asubstantial part of the inductance of the resonant applicator.Preferably however, each of the plunge'rs 17C is effectively shunted, asshown in Fig. 5, by a circular array of conductive straps 49C connectedat their lower ends to the upper face of electrode 160. The upper endsof straps 49C engage and are preferably fastened to the periphery ofcylinder 13C. Each peripheral array of straps defines a substantiallyfield-free space or compartment for the associated plunger 17C, orequivalent electrode-moving element. Such straps together with theassociated cylinder afford a low resistancev path for the heavycirculating currents between the upper wall of the tunnel and the hotelectrode 16C and no appreciable current flows along the plunger.

Each cylinder 18C and its associated strap members 49C jointly form acolumnar inductance element of the applicator and the row of suchelements forms an inductance structure, elongated in adirectiontransverse to the current flow, between electrode 16C and thetop wall of the applicator. housing 11C.

The applicator 10D, Fig. 6, is similar to that of Figs. 4, Sin that theverticalcylinders 18D are within the applicator housing 11D. In thismodification however, the row of cylinders 18D is within a hollow fin13D defined by the two conductive webs or sheets 49D disposed onopposite sides of the cylinders and extending lengthwise of theelongated electrode and housing beyond the and cylinders of the row. Theopposite ends of each of the sheets 49D are respectively attached, as bybars 56D, 57D, to the upper wall of the housing 11D and to the hotelectrode 16D. The spaced sheets 49]) thus define an elongated fininductance within which the cylinders 18D and their plungers 17D aredisposed in substantial isolation from the high-frequency magnetic andelectric fields of the resonant applicator. The cylinders and plungersare shielded from the magnetic field by the spaced sheets 49Dand fromthe electric field by the extension of electrode 16D across theirinwardly extending ends. In this modification, neither the cylinders northeir plungers form part of the applicator inductance and they carry nopart of the circulating current, In other respects the applicator 10D issimilar to those previously discussed and need not be further described.

In the modified press-applicator 16E shown in Fig. 7, the positions ofthe stationary and movable platens are interchanged, as compared topreceding modifications, so that the vertical cylinders 18E, orequivalent electrodeactuating devices, may be disposed beneath theapplicator housing llE in the press foundations. As in the precedingmodifications, the cylinders 18E, or equivalent, extend externally ofthe housing HE from one of its side walls. In any or thesemodifications,the stroke of the side plungers may be small and theapplicator adapted for awide range in width. of fan edge-bonding load byprovision of manually adjustable pressure screws 50E along the oppositeside wall.

The stationary electrode 16B is formed by the lower wide and elongatedface of a beam 13E attached to the upper wall and forming part of thepress frame. The movableelectrode 15E of corresponding length and widthis supported by orupon two spaced rows of vertical plungers or rams1713. These rows extend lengthwise of the electrode 1513. Each of theelongated sides of the movable electrode 15E is connected to the bottomof housing 11E or to the side walls, below the uppermost position of themovable electrode by a series of conductive straps or by a wide flexibleweb, generically illustrated by flexible members 49E, so to define asubstantially field-free space within which the electrode-operatingplungers 17E, or equivalent, are disposed, generally as in Figs. 2, 3and 6.

The capacitance of the resonant applicator E is primarily that betweenthe opposed faces of electrodes E, 16E. The web or beam 13B is asignificant part of the total applicator inductance and substantiallyall of the remainder consists of the inductance of the straps or webs49E.

As also in the modifications previously described, the free verticaledges of the inductance structure should be spaced from the ends ofapplicator housing when such ends are closed, either completely orpartially, in order to leave an unobstructed path for the high-frequencymagnetic flux encircling such structure. To provide for insertion orremoval of work, either or both ends of the applicators ltlA-IOE may beleft open or doors or covers may be provided to minimize interference tosensitive radio-receiving equipment. Closed applicators may bepressure-tight so that dielectric heating may be etfected atsubatmospheric or superatmospheric pressures. Partial end closuresleaving an unobstructed path for insertion or removal of the work fromeither end of the applicator or for continuous flow of the work throughthe applicator, as by a conveyor belt, such as indicated at 63D in Fig.6, may be provided.

The loop 51E may be provided for excitation of the applicator 1013.

In the modification shown in Fig. 8, the fin structure 13F of applicator10F is hollow to form a completely enclosed shielded compartment or boxin which the oscillator tube 25F and associated circuit components maybe disposed in isolation from the high-frequency magnetic fieldencircling the fin and from the high-frequency electric field betweenthe heating electrodes 15F, 16F. In any of the preceding modifications,the space defined by the hollow fin structure may be completely enclosedto insure the space is entirely field-free.

in the particular oscillator circuit 24F shown in Fig. 8, thegrid-excitation is provided by loop 71F which extends into the fluxspace of the applicator on the side of fin structure 13F which isopposite from the anode loop 51F. Thus, there is no appreciable couplingbetween the loops 51F, 71F except that afforded by the high-frequencymagnetic fiux encircling the fin inductance 13F. The loops 51F, 71F areso poled or connected that the induced grid voltage is of proper phasefor sustained generation of oscillations at a frequency principallydetermined by the capacity between the heating electrodes 15F, 16F andthe inductance of the fin 13F. The condenser 27F and resis tor 28Fwithin the hollow fin 13F are for deriving a directcurrent grid-biasingvoltage from the radio-frequency grid current of the oscillator tube.The condenser 61F within fin 13F is a radio-frequency by-pass ordirect-current blocking condenser.

As in Fig. 9, later described, a load tray may be provided for insertionor removal of work 19F, the hot electrode 16F being at convenient heightfor manual loading of plastic preforms or the like; or an insulatedconveyor belt moving along or above the elevated hot electrode 16F maytransport work through the applicator between the electrodes 15F, 16F.In the latter case, the substantial spacing between the edges of the-hot electrode and the applicator side walls prevents arcing as the workobjects pass into or out of the interelectrode space. The applicator 10Fmay of course be inverted.

The inductance fin structure of tunnel applicators such as hereindisclosed need not be symmetrically located in the applicator housing;it maybe otfset,,asin Fig.9, to

atford space in the housing which may beused for enclosing of theoscillator tube and other circuit components. It is only necessaryeffectively to isolate such other oscillator elements from the tunnelflux and to provide an adequate path for encirclement of the finstructure by the high-frequency magnetic flux of the resonantapplicator.

More specifically, the oscillator tube 256, associatedoscillator-circuit components, and the system components such as blower686 may be disposed in shielded compartment 646 within the housing 11Gof applicator 10G. Other shielded compartments 69G within applicatorhousing 11G may also be provided for meters, control relays and otherpower-supply equipment to complete a selfcontained mobile unit suitedfor heating of plastic preforms 19G and for like uses requiring powersof the order of a few kilowatts.

As shown in Fig. 9, such compartments are located to leave a free pathfor high-frequency magnetic flux to encircle the hollow fin structure136 which extends from the bottom wall of the applicator housing 11Gwith the hot electrode 16G at its upper end so that the top wall of thehousing may serve as the associated cold electrode 15G, or so that anadjustable upper electrode (not shown) connected to the top wall may beraised or lowered to accommodate preforms of different height or to varythe effective power-factor of the loaded applicator and so vary the loadreflected into the anode circuit of the tube.

The load tray G for insertion and removal of the preforms, or otherwork, may be comprised of a thin sheet of low power-factor material,such as glass or glass fibre-board impregnated with silicone resin,fastened to a metal door or panel 55G which completes closure of theapplicator housing when the load tray is inserted. Alternatively, thetray may be made of metal, suitably insulated from the metal door orpanel 55G. Tray 706 may simply slide upon and be supported by the hotelectrode 166 and may be wholly retractable from the applicator housingfor loading or removal of work.

A preferred oscillator system for this resonant applicator as well asthe others herein described is the oscillator circuit 24G shown in Fig.9 and more fully described and claimed in my aforesaid applicationsSerialNos. 138,628, and 419,633. It automatically maintains propergridexcitation of the oscillator tube under wide variation of work-loadconditions during a run or for-widely difierent work-load conditions ofdifferent heating runs.

The loop 51G inductively couples the resonant applicator to the anodecircuit of the oscillator tube. The grid of the tube is connected toheating electrode 16G through a capacitor 59G which usually is of highreactan'ce compared to the etfective grid-cathode capacity 62G of thetube. In the arrangement shown in Fig. 9, the lead from grid-capacitor596 extends through an insulator in the top wall of compartment 64Gdirectly to the heating electrode'16G. In'such case, the maximuminterelectrode voltage, i. e., the voltage between the heatingelectrodes, is applied to capacitors 59G, 62G in series. Should this beexcessively high, the capacitor 596 may be electrically connected to theelectrode .1 6G by a lead extending through the side wall of compartment64G to a selected point along the fin inductance 13G. In such case, afraction of the total voltage between the heating electrodes is appliedto series capacitors 59G, 62G, the fraction being smaller and smaller asthe connection to capacitor 596 is shifted farther and f rther alonginductor 13G away from electrode 16G. Condenser 27G and resistor 28G areforideriving a direct-current grid biasing voltage from the rectifiedgrid current. RFC designates a non-resonant choke which provides a highimpedance from the grid to the network 27G, 28G. The cathode of thetube, so far as the generated oscillations are concerned, may begrounded directly or through by-pass condensers (not shown).

When, as above pointed out, it is desirableto tap onto thefin structurefor grid-excitation, the hollow fin may be cesslv'e vol ge gradientalong the heating electrode. if short grid-lead length were obtained bydisplacing the tin toward 'the oscillator compartment, the voltagegradient along the heating "electrode might become excessive withlargeblectrodes. Excessive "grid-lead length is to be voided because"itfcaujs'e's instability 'ofjosc'illation, operation. Thus 'hollow fi nconstruction has the additional advantages offaciiitating tappingthegridconnection onto the fin structure without incurring attendantdisadvantages, such as above mentioned, 'Which might otherwise e l its tt The radio-frequency potential difference between the applicatorelectrodes may be adjusted to any desired value within a wide range byadjustment of the mutual inductance between "the anode loopfand theresonant applicator. In the particular arrangement "shown in Fig. 9,this is accomplished by the adjustable shorting bar 676: alternativlysilqh'a'djustrn'ent of the mutual inductance may be 'efiec'ted byrotating theanode loop 51E (Fig. 7) or by any of the various other ways'shown in my aforesaid applications Serial NOS. 138,628, and "419,633..As the mutual inductance 'is varied in a direction to increase thepotential of the hot electrode, the capacitor 596 is varied in sense toprevent excessive grid excitation. As the capacity of capacitor 59G ismuch less than the effective input capacity 626 of the oscillator tube,the radiofrequency (R. F.) potential of the hot" electrode may be manytimes higher than the grid-potential, thus permitting thehigh electrodepotential required for dielectric heating without exceeding a safegrid-potential. More particularly, the R. F. -grid-potential is always afraction of the R. F.potential-difterence between the heating electrodesand is inverselyproportional to the ratio of the total reactance or theseries-connected capacitors 59G, 62G to the reactance of the effectiveinput capacity 62G of the oscillator tube (alone or additive to thecapacity of an external shunt condenser).

With the m'utu'al inductance or coupling of the applic'ator and anodecircuit adjusted to providethe desired R. F. voltage of electrode 16G,or equivalent, and with capacitor 596 preadjusted or preset for propergridexcitation, the capacity 626 has an eifective value which (as fullyexplained in my aforesaid applications) inherently varies with theapplicator load so that the ratio of the, two reactances of thecapacitor voltage-divider 59G, 62G varies automatically with load'and inproper sense to stabilize the grid excitation.

The maximum heating-electrode voltage occurs under a condition ofoptimum coupling (between the resonant applicator and the anode circuit)for which the eifective resistance Rh, reflected into the anode circuitof the oscil' lator tube from the applicator, is equal to the efiectiveanode resistance Rp- The corresponding optimum mutual inductance (M) isgiven by the equation Jana.

where Tf=operating frequency Rr=efective series resistance of theapplicator For reasons herein briefly stated but more fully dis cussedin my aforesaid applications, the mutual-inductance 'betweenloop 516, orequivalent, and theresonant applicator is supraoptimum, i. e., higherthan optimum. In such case, the heating electrode voltage does notsubstantially vary with change in the loaded Q of the applicator aswould occur, for example, upon change of the characteristics ofa'dielectric load during'its heating or, in a conveyor-fed applicator,upon change'in the number of. load objects moving between the heatingelectrodes.

The hollow fin construction facili-ties 'supraoptimuni assumeicoupli'n'g, for any given size of anode coupling loop (51E, 51F, 516),in that by properly dim'ensioning the fin "the nx density through theloop may be increased. specifi c l by increasing the cross-sectionalarea of the fin structure, greater concentration of flux in the vicinityof the loop may be obtained. The hollow fin construction thus permitssupraoptimum coupling to be obtained even when the loop area, forvarious considerations, should be small.

The operational advantages in dielectric-heating of suprnoptimumcoupling and the capacity voltage-divider arrangement are more fullydiscussed in my aforesaid applications.

Theapplicators of Figs. '8 and 9, as described or heating of plasticpreforms, need supply only a few kilowatts (2 to 5 for example) whereasthe applicators for heating of wallboard sheets, face-bonding oflaminated sheets, etc., such as exemplified by Figs. 1 to 7, supply highpower, well over a hundred kilowatts, and employ heating electrodes manyfeet in length and breadth. In all of them, the F. power losses in theresonant applicator are a relatively small percentage of the total R. F.power delivered to the applicator. Byway of specific example, a tunnelapplicator having an unloaded Q of 2750, housing height, length andWidth or" approximately 3, '12 and 8 feet respectively, and a 5foot by10-foot electrode of 370 micromicrofarads capacity, and operating at afrequency of i4 megacycles with a peak electrode voltage of 32,060volts, delivered 14-8 kilowatts in heating a load having a power-factorof 0.9% with a power loss of only '6 kilowatts in the resonator.Agenerator using a conventional load circuit having an unloaded Q of 200and delivering the same power (148 kw.) to an identical load would beforced to supply 83 kilowatts of Wasted power to the'load circuit.

Additional advantages of the hollow fin constructions of these resonanttunnel applicators are that they provide compartments within which theoperating mechanism for the heating electrodes, the oscillator tube, orother system components or their connections may be disposed insubstantially complete isolation from the intense highfrequency magneticand electric fields existent in the applicator housing. Moreover, thehollow fin may be of any desired cross-sectional outline, for example,round, rectangular, octagonal or the like, and need not be symmetricallylocated with respect to the outline of the associated electrode (see,for example, Fig. 9). Thus with economy of fin metal and withflexibility of the fin, when desired, the perimeter of the fin may besuitably large to obtain an inductance of high Q" and to minimize orcontrol voltage gradients along large electrodes without need forstubbing and its attendant complications.

Among the important and singular advantages of resonant applicators suchas herein described is that they have made possible, on a commercialscale, the efiicient and uniform heating of large sheets or masses ofdielectric material of very low power-factor, i. e., of 1% and less, sopermitting the application of dielectric heating equipment for suchpurposes as heating or drying of pulp wallboard, foam rubber, pure gumrubber and the like. With conventional coil circuits or applicators, thepercentage of power dissipated as circuit losses is excessively high forpower-factors lower than 1%. Furthermore, with conventional coilcircuits or applicators, operation at higher frequencies to obtainefiicie'nt heating at voltages low enough to prevent arcing, and withelongated electrodes of relatively large area to accommodate largesheets, panels and the like, requires stubbing which, aside fromdilficnlties of adjustment, is cumbersome and so reduces the unloaded Qof the heating circuit or applicator that heating of low power-factorloads is impractical.

The use of microwave or very high-frequency generators using waveguides,concentric lines, and conventional resonant cavities requiringcomplete'enclosureand axial symmetry as applicators is unsatisfactoryfor applications of dielectric heating involving work of largerectangular area and substantial thickness because of nonuniform heatingdue to standing waves, localizing of heat at or near the surface of thework, and the low-power, lowvoltage limitations of such equipment. Incontrast thereto, resonant applicators of the kind herein described maybe designed and dimensioned in accordance with principles herein setforth so as to provide, when desired, elongated heating electrodes oflarge area between which the electric field may be made substantiallyuniform, without stubbing, by elongation of the associated inductancestructure, and so that operation of the applicators without stuhbing maybe carried on at frequencies which are suitably high or safe,satisfactory heating despite the high electrode capacity required fordielectric loads of great length and area.

In the examples given, and as usually is desirable in resonantapplicators such as herein described, the length of the fin inductancein the direction of current flow is very substantially less than aquarter-wavelength at the operating frequency, ordinarily being evenless than an eighth-wavelength, and also is less than the maximumdimension of the electrode where elongated electrode structures areemployed.

Moreover, when it is desirable to secure substantial uniformity ofvoltage gradient over the whole of the heating electrode area, this canbe accomplished by making at least one face dimension of the electrodestructure very much smaller than a wavelength at the operatingfrequency, and, when an elongated electrode is used, the fin inductanceis elongated in the direction of elongation of the electrode so that thelength of the fin is very close to, or equal to, the length of theelectrode. Still more particularly, in the case of an elongatedelectrode, its projection, if any, beyond the elongated fin should besubstantially less than an eighth-wavelength in the direction ofelongation. Also, the amount of projection, if any, of the electrodebeyond the fin inductance in the direction at right angles to thedirection of elongation of the fin inductance should be substantiallyless than a quarter-Wavelength and usually even less than aneighthwavelength. For satisfactory commercial operation, it usually isdesirable that the half-width of the electrode, taken normal to thedirection of elongation of the fin inductance, be substantially lessthan an eighth-wavelength. By following the principles above set forth,it is possible, with applicators of the kind to which the presentinvention relates, to employ, when desired, electrodes of large area andrelatively much as one-eighth-wavelength or more at the operatingfrequency, while obtaining substantial uniformity of voltage gradientover the electrode area without stubbing.

In batch operation, it usually is desirable to maintain substantialuniformity of voltage gradient throughout the heating electrode area.However, in applicators employing conveyors for continuous feeding ofmaterial, it frequently is not so important to maintain uniformity ofvoltage gradient along the electrode structures in the direction ofconveyor travel, thus the electrode structures may be quite long in suchdirection of conveyor travel, but for reasons set forth above, thedimension of the electrode structure at right angles to the direction ofconveyor travel usually should be maintained small relative to awave-length at the resonant frequency of operation of the applicator.Alternatively, where it is desired to employ an electrode ofrelativelylarge dimension in the direction at right angles to thedirection of conveyor travel, while maintaining substantial uniformityof voltage gradient in that direction, the fin inductance may beextended in that direction in accordance with principles above setforth.

What is claimed is:

l. A resonant high-frequency electric heating applicator of thereentrant type comprising a housing having great 'length, i. e.,equivalent to as r electrically conductive walls, electrode structurescoopera tive to provide an electric field space within said housing forreceiving in said field space between said electrode structures materialto be heated, a fin-like inductance structure projecting into theinterior of the housing and electrically connected at opposite endsrespectively to one of said electrode structures and to wall structureof the housing, means including said wall structure completing aresonant circuit which includes said inductance structure and thecapacity between said electrode structures, said wall structureproviding a low-resistance, low-reactance path as a part of saidcircuit, and coupling means disposed within the housing in couplingrelation with said fin-like inductance structure for supplyinghigh-frequency energy to said resonant circuit for the heating of saidmaterial disposed between said electrodes, said housing serving as ashielding enclosure to confine the magnetic field encircling theinductance structure and the electric field produced between theelectrode structures, and said inductance structure being hollow andshielded at one end by said one electrode structure to define therein asubstantially field-free space effectively isolated from said magneticand electric fields.

2. A resonant high-frequency heating applicator as in claim 1 in whichsaid hollow inductance structure comprises two members extending inspaced-apart substantially parallel relationship to each other.

3. A resonant high-frequency applicator as in claim 2 in which each ofthe inductance structure members is continuous.

4. A resonant high-frequency applicator as in claim 2 in which each ofthe inductance structure members comprises a row of conductive straps.

5. A resonant high-frequency heating applicator as in claim 1 in whichauxiliary applicator equipment is disposed within the field-free spacedefined by said hollow inductance structure.

6. A resonant high-frequency heating applicator as in claim 5, in whichsaid one electrode structure is movable for variation of the spacingbetween the electrode structures, and in which auxiliary equipmentwithin the hollow inductance structure includes means for effecting suchmovement of said one electrode.

7. A resonant high-frequency heatiru applicator as in claim 1, in whichsaid hollow inductance structure is at least in part extensible forvariation of the spacing between the electrode structures, and in whichactuating mechanism for varying the spacing between the electrodestructures is shielded from said fields, at least part of said mechanismbeing within the field-free space defined by said hollow inductancestructure.

8. Apparatus for high-frequency electric heating of dielectric materialscomprising a resonant applicator including a housing having electricallyconductive walls and electrode structures cooperative to provide anelectric field space within said housing for receiving in said fieldspace between said electrode structures material to be heated, aninductance structure within the housing and electrically connected atopposite ends respectively to one of said electrode structures and towall structure of the housing, means including said wall structurecompleting a resonant circuit which includes said inductance structureand the capacity between said electrode structures. electricallyconductive structure providing a shielding compartment within saidhousing, and an oscillator system for exciting said applicator andhaving components disposed within said shielding compartment.

9. Apparatus as defined in claim 8, in which the said inductancestructure is hollow and defines said shielding compartment.

10. Apparatus as defined in claim 8, in which a coupling loop extendsfrom said shielding compartment for traverse by the high-frequencymagnetic field encircling said inductance structure, said loop beingincluded in an elecarsena s nods circuit of an oscillator tube withinsaid shielding compartment. V

11. A as natit high-frequency heating applicator of the reentrant typecomprising spaced heating electrodes for receiving between them materialto be heated by the electric field produced between them, a housinghaving conductive walls of low 'reactance, a fin-like inductancestructure extending inwardly of said applicator from wall structure ofsaid housing to one of said electrodes, said inductance structure beinghollow to define therein a field-free space efiectively isolated fromthe high-frequency magnetic field about said inductance structure andsaid one of said electrodes being disposed across the inwardly extendingend of said inductance structure to shield the interior thereof from thehigh-frequency electric field between said heating electrodes, andcoupling means disposed within said housing and in coupling relation tosaid inductance structure for supplying highfrequency energy to aresonant circuit formed by the inductance of said inductance structureand the capacitance between said spaced heating electrodes for heatingsaid material disposed between them. 7

12. An applicator for the heating 'of dielectric work comprising anelectrically conductive housing, inductance structure therein comprisinga fin inductor projecting into the interior of said housing, spacedelectrode structures cooperative to provide electric field space withinthe housing for receiving therein material to be heated by'the electricfield between said electrode structures, at least one of said electrodestructures being movable to permit variation in the spacing between saidelectrode structures, said one electrode structure being disposed at theinwardly projecting end of said fin inductor in spaced relation to wallsof the housing and being electrically connected with wall structure ofthe housing through said fin inductor, said wall structure providing alow re sistance, low reactance path completing a resonant circuit whichincludes said inductance structure and said electrode structures and thefrequency of which is predominantly determined by the inductance of saidinductance structure and the capacitance between said electrodestructures, said fin inductor being hollow and said one electrodestructure at said inwardly projecting end of said fin inductor shieldingthe interior thereof to define within said fin inductor a substantiallyfield-free space effectively isolated from said electric field and fromthe magnetic field about said fin inductor, and mechanism operable toeffect movement of said one electrode, at least part of which mechanismis disposed in said field-free space within said fin inductor.

13. An applicator for high-frequency electric heating of dielectricmaterial comprising spaced cooperative electrodes for receivingtherebetwcen the material to be heated, at least one of said electrodeshaving elongated rectangular configuration and being of large area, ahollow inductance structure electrically connected at one end to saidone electrode and having rectangular transverse cross-section shieldedby, and of area substantially less than the area of said one electrode,and an electrically conductive housing of rectangular cross-sectionenclosing said structure and the space between said electrodes andhaving walls which are in spaced relation to said one electrode, theother end of said inductance structure being electrically connected towall structure of said housing, said applicator being resonant at afrequenc'y predominantly determined by the inductance of said structureand the capacity between said electrodes, the length 'ofsaid inductancestructure between said wall structure and said one electrode beingsubstantially lessthan the length of said one electrode, and saidinductance structure having such cross-sectional area that the distancetherefrom to each edge of said one electrode is substantially less thanone-eighth-wavclength at said frequency.

-14. A resonant high-frequency applicator as in claim i: in

12 13, in which the said one electrode is movable for variation of thespacing between said electrodes, and including mechanism for effectingsuch movement, at least part of which mechanism is disposed within theshielded field=free space inside of said hollow inductance structure.

15. A high-frequency heating applicator comprising walls forming anenclosure, walls forming a shielding compartment within said enclosure,said shielding compartment extending substantially across said enclosurewith space around said compartment between the walls of said compartmentand the walls of said enclosure, high-frequency generating equipmentenclosed within said compartment, an electrode conductively connected tothe upper wall of said compartment and forming a capacitor with theopposing wall of said enclosure, and at least one inductive loopextending into said open space for development between said electrodeand said opposing wall of a high-frequency, high-voltage electric field,the walls of said compartment and their conductive electrical connectionwith said electrode forming the inductance of an oscillatory circuitexcited through said loop from said generating equipment, thecapacitance of said circuit being determined by the spacing between saidelectrode and said opposing wall of said enclosure.

16. A resonant applicator comprising a pair of elec* trodes havihgsubstantial breadth and greater length and which are supported in spacedrelation one from the other, elongated conductive members spaced fromeach other in the direction of the width of one of said electrodes andextending in the direction of elongation thereof, each of said membersbeing electrically connected at one elongated end to said one of saidelectrodes, a conductive-walled housing enclosing said members and thespace between said electrodes, and means including wall structure ofsaid housing electrically connecting the other elongated ends of saidconductive members to the other electrode and forming a confined pathfor the highfrequency magnetic field about said members.

17. A resonant applicator for high-frequency electric heating ofdielectric materials comprising an electrically conductive housing,inductance structure disposed in said housing and including at least oneinductive element of hollow cross-section and relatively largecross-sectional area, spaced electrode structures cooperative to provideelectric field space within the housing, one of which electrodestructures is disposed at one end of said inductive element in spacedrelation to wall structure of the housing and electrically connectedwith said wall structure through said inductive element, means includingsaid wall structure completing a resonant circuit which includes saidinductance structure and said electrode structures, said housing servingasa shielding enclosure to confine the electric field produced betweensaid electrode structures and the magnetic field-encircling saidinductance structure, and a conveyor for transporting material throughsaid electric field space, said one electrode structurehavingsubst-antially greater area than the cross-sectional area of saidinductive element and extending beyondsaid element on both sides, in thedirection at right angles to the direction of conveyor travel, adistance which is substantially less than one-eighth-wavelength at theoperating frequency of the resonant applicator.

18. A resonant high-frequency heating device comprising a tunnelapplicator having conductive walls whose resistance and'reactance arenegligible, a high-frequency heatingelectrode within said applicator inspaced relation to all walls thereof, a second electrode providing withsaid heating electrode substantially all the capacitance of'saidapplicator, means for moving said heating electrode toward and from oneof said walls'for varying the potential gradient through a load beingheated, said means including actuating metallic members conductivelyconnected to said heating electrode and tothe opposite one of saidwalls, a metallic inductor surrounding said actuating members to isolatesaid actuating members from high-frequency fields within said applicatorand to provide substantially all the inductance of said resonant device,and means for electrically connecting the opposite ends of said inductorrespectively to said heating electrode and to said opposite wall of theapplicator, said connecting means at least at one end of the inductorcomprising flexible conductors.

19. A resonant high-frequency heating device comprising a tunnelapplicator having conductive walls whose resistance and reactance arenegligible, high-frequency heating electrode structure within saidapplicator in spaced relation to all walls thereof, a hollow metallicinductor structure attached at one end to said electrode structure andat the opposite end to a wall of said applicator, said inductorproviding substantially all the inductance of said tunnel applicator,and means for moving said heating electrode structure toward and from awall of said applicator for varying the potential gradient through aload to be treated, said means comprising metallic members conductivelyattached to one of said structures within the field-free boundary ofsaid inductor structure and conductively engaging and passing through anopposite wall of said applicator.

20. A resonant high-frequency heating applicator comprising an enclosurehaving conductive walls whose resistance and reactance are negligible,heating electrode structure within said enclosure in spaced relation towalls thereof, inductance structure comprising a hollow inductiveelement electrically connected at one end to said electrode structureand electrically connected at an opposite end to said enclosure, theinterior of said element comprising a field-free space, said inductancestructure providing substantially all the inductance of said applicator,means for moving said electrode structure toward or away from a load tobe heated, said means comprising at least one metallic memberconductively attached to one ductor, said wall structure of saidstructures within said field-free space of said inductive element andpassing through one of said walls within an area encompassed by saidinductive element.

21. An applicator for the heating of dielectric work comprising anelectrically conductive housing, inductance structure therein comprisinga fin inductor projecting into the interior of said housing, and spacedelectrode structures cooperative to provide electric field space withinthe housing for receiving therebetween material to be heated bytheelectric field between said electrode structures, one of saidelectrode structures being disposed at the inwardly projecting end ofsaid fin inductor in spaced relation to walls of the housing andelectrically connected with wall structure of the housing through saidfin inp-roviding a low resistance, low reactance path completing aresonant circuit which i-ncludes said inductance structure and saidelectrode structures and =the frequency of which is predominantlydetermined by the inductance of said inductance structure and thecapacitance between said electrode structures, said fin inductor beinghollow and said one electrode structure at said inwardly projecting endof said fin inductor shielding the interior thereof to define therein asubstantially field-free space effectively isolated from said electricfield and from the magnetic field about said fin inductor.

References Cited in the file of this patent UNITED STATES PATENTS2,107,387 Potter Feb. 8, 1938 2,124,029 Conklin et a1 July 19, 19382,215,582 Goldsti-ne Sept, 24, 1940 2,218,223 Usselman et al Oct. 15,1940 2,465,102 Joy Mar. 22, 1949 2,504,109 Dakin et al. Apr. 18, 19502,708,703 Cunningham et .al May 17, 1955

